In recent years, a marketing product using a packaging bag such as retort pouch and the like which can keep foods as contents, is required to avoid alterations of food tastes and of qualities during a period from producing the marketing product until eating by customers. Therefore, a packaging material having a layer structure laminated by an aluminum foil or aluminum vapor deposition layer so as to be the internal side of a bag against a base material film provided with back printing, is used for such packaging bag and the packaging bag is devised to obtain a light shielding provided by giving said aluminum foil or aluminum vapor deposition layer in the layer structure.
Also, since retort foods and the like provided by the above mentioned packaging bag are widely and commonly distributed and consumed, the packaging bag made by the above mentioned packaging material may come into circumstances to be disposed as wastes, and in case of a thermal disposal of the packaging bag, the fact that metal portion in the packaging material remains as residues in an incinerator, is acknowledged as a problem.
From this point of view, in order to retain a light shielding property without a use of metal such as aluminum, a light shielding layer made by way of printing is applied to form in the layer structure of the packaging material. And since a coloring is not suitable for the marketing product when a black color ink having high light shielding property is used in case of forming said light shielding layer by printing, selecting a color having the light shielding function as said shielding layer and simultaneously being suitable color for food containers, has been tried.
Meanwhile, as the above mentioned packaging bag such as retort pouch is in the form that the packaging materials are laid on each other so that facing peripheral edge parts are heat sealed, the packaging bag before packing contents is in the form having an open state fill opening at one side with facing non-sealed position which is intended for heat seal. And the packaging bag having the open end at one side as the fill opening is set to the filling machine to fill contents from said fill opening, and subsequently, after heat seal the fill opening, as described above, the peripheral position is heat sealed so that a form of the market product having seal contents closely is obtained.
Also, while filling, since splashes and the like of contents are adhered on facing surfaces of position intended for heat seal of the fill opening and since heat seal may possibly be done with keeping the state, after the heat seal is done at the fill opening, to the position which was formerly the fill opening, the existence of defective seal and the like if the bitten seal which caught foreign matters or the insufficient seal occurs, is inspected visually as holding the heat sealed fill opening up against light and the like.
However, for the packaging bag applying the packaging material comprising aluminum foil in the layer structure, since the packaging material itself has extremely high light shielding property, an interface state between seal sides which were formerly the fill opening position, can not be so as to be seen by holding up to light and a detection of the defective seal such as the bitten seal or the insufficient seal is difficult and also, the detection of the defective seal is difficult as well for even packaging bag which was applied the packaging material having the light shielding layer by printing as described above.